1. Technical Field
The embodiments described herein relate generally to radio Frequency Identification (RFID), and more particularly to RFID protocols and methods that can be used to avoid collisions among a plurality of interrogators in a single environment.
2. Related Art
Radio-frequency identification (RFID) is an automatic identification method, relying on storing and remotely retrieving data using devices called RFID tags or transponders. An RFID tag is an object that can be applied to or incorporated into a product, animal, or person for the purpose of identification using radio waves. The data is retrieved from the RFID tags using a reader or interrogator.
Most RFID systems use a modulation technique known as backscatter to enable the tags to communicate with the reader or interrogator. In a backscatter system, the interrogator transmits a Radio Frequency (RF) carrier signal that is reflected by the RFID tag. In order to communicate data back to the interrogator, the tag alternately reflects the RF carrier signal in a pattern understood by the interrogator. In certain systems, the tag can include its own carrier generation circuitry to generate a signal that can be modulated with data to be transmitted to the interrogator.
Most RFID tags contain at least two parts. One is an integrated circuit for storing and processing information, modulating and demodulating a (RF) signal, and other specialized functions. The second is an antenna for receiving and transmitting the signal. As the name implies, RFID tags are often used to store an identifier that can be used to identify the item to which the tag is attached or incorporated. But in today's systems, a RFID tag can contain non-volatile, possibly writable EEPROM for storing additional data as well.
RFID tags come in one of three types: passive, active, and semi passive. Passive RFID tags have no internal power supply. The minute electrical current induced in the antenna by the incoming RF signal from the interrogator provides just enough power for the, e.g., CMOS integrated circuit in the tag to power up and transmit a response. Most passive tags signal by backscattering the carrier wave from the reader. This means that the antenna has to be designed both to collect power from the incoming signal and also to transmit the outbound backscatter signal.
Passive tags have practical read distances ranging from about 10 cm (4 in.) (ISO 14443) up to a few meters (Electronic Product Code (EPC) and ISO 18000-6), depending on the chosen radio frequency and antenna design/size. The lack of an onboard power supply means that the device can be quite small. For example, commercially available products exist that can be embedded in a sticker, or under the skin in the case of low frequency RFID tags.
Unlike passive RFID tags, active RFID tags have their own internal power source, which is used to power the integrated circuits and to broadcast the response signal to the reader. Communications from active tags to readers is typically much more reliable, i.e., fewer errors, than from passive tags.
Active tags, due to their on board power supply, also may transmit at higher power levels than passive tags, allowing them to be more robust in “RF challenged” environments, such as high environments, humidity or with dampening targets (including humans/cattle, which contain mostly water), reflective targets from metal (shipping containers, vehicles), or at longer distances. In turn, active tags are generally bigger, caused by battery volume, and more expensive to manufacture, caused by battery price.
Many active tags today have operational ranges of hundreds of meters, and a battery life of up to 10 years. Active tags can include larger memories than passive tags, and may include the ability to store additional information received from the reader, although this is also possible with passive tags.
Semi-passive tags are similar to active tags in that they have their own power source, but the battery only powers the microchip and does not power the broadcasting of a signal. The response is usually powered by means of backscattering the RF energy from the reader, where energy is reflected back to the reader as with passive tags. An additional application for the battery is to power data storage.
The battery-assisted reception circuitry of semi-passive tags leads to greater sensitivity than passive tags, typically 100 times more. The enhanced sensitivity can be leveraged as increased range (by one magnitude) and/or as enhanced read reliability (by reducing bit error rate at least one magnitude).
The enhanced sensitivity of semi-passive tags place higher demands on the interrogator concerning separation in more dense population of tags. Because an already weak signal is backscattered to the reader from a larger number of tags and from longer distances, the separation requires more sophisticated anti-collision concepts, better signal processing and some more intelligent assessment of which tag might be where.
The collision problem can also affect the readers, or interrogators, i.e., it is possible in certain environments for multiple interrogators to transmit at the same time, which can create communication problems within the system. A RFID interrogator can be considered to be operating in a subset of the following categories:
a single-reader environment;
a multiple-reader environment;
and a dense reader environment.
With respect to the collision problem, in a single-reader static environment, there is no real need for reader arbitration, since there is no other reader to compete against. But, “plug-and-play” flexibility can still be incorporated in the event that another interrogator (or more) is temporarily or permanently added, resulting in a multiple-reader environment.
In multiple, or dense reader environments, it is possible that two or more readers will transmit at the same time. For the purposes of this discussion, a multiple-reader environment is defined to be a single RFID environment with up to 25 readers present. A larger number of readers is considered to form a dense reader environment. Thus, multiple and/or dense reader environments can produce communication problems due to two or more readers transmitting at the same time.